rt-ac66; rt-ac66u; rt-ac66r; rt-ac66 repeater

Hi guys. I have 2x ASUS RT-AC66R. First one is set as router and second one as repeater. Repeater is rebooting every few minutes and can't figure it out why. Firmware is last official from asus 3.0.0.4.380_3264. I had Merlin's firmware before on repeater but had same issue. So I think it's not...